This invention relates to static convergence apparatus for cathode ray tubes for color television receivers.
Color display systems such as utilized in color television receivers include a cathode ray tube in which three electron beams are modulated by color-representative video signals. The beams impinge on respective color phosphor areas on the inside of the tube viewing screen. To accurately reproduce a color scene, the three beams must be substantially converged at the screen at all points on the raster. The beams may be converged at points away from the center of the raster by utilizing dynamic convergence methods or self-converging techniques, or a combination of both. Regardless of the methods utilized to achieve convergence while the beams are deflected, some provision must be made to statically converge the undeflected beams at the center of the screen. Static convergence devices are necessary because the effect of tolerances in the manufacture of electron beam guns and their assembly into the cathode ray tube neck frequently results in a static misconvergence condition.
Some static convergence devices, used, for example, in in-line beam color kinescopes, converge the outer beams of the three in-line beams onto the central beam by means of four and six pole magnetic field ring pairs, producing opposite and like movements, respectively, of the outer beams, as described in U.S. Pat. No. 3,725,831, granted to R. L. Barbin. Other static convergence devices, such as described in U.S. patent application Ser. No. 698,464, filed on June 21, 1976, for J. L. Smith, provide for individual motion in any direction of each outer beam of three in-line beams by means of two eccentrically located magnetic ring pairs.
For each of the above static convergence devices, the motion described by a given electron beam is controlled in magnitude by equal and opposite rotation of each ring member of the appropriate ring pair. For example, a static convergence device described in the aforementioned Smith application has a magnetic ring pair comprising illustratively a six-pole configuration with an internal zero magnetic field point located between the center and one outer beam to effect motion of the other outer beam.
For relatively poorly converged cathode ray tubes, where one outer beam is at a relatively great distance from the center beam, a large magnitude motion of the outer beam is required in order to converge it to within an acceptable distance of the center beam. To accomplish such large magnitude motions, the ring members of the appropriate ring pair are rotated relative to each other so that the magnetic field of each member is aligned additively with the field of the other rather than in a cancelling orientation. The maximum resultant magnetic field, corresponding to maximum motion of the electron beam, occurs when a North pole of one ring member is directly aligned with the North pole of the other ring member.
Minimum motion of the electron beam occurs when a North pole of one ring member is aligned with a South pole of the other ring member, the fields of one cancelling the effect on the electron beam of the other. This minimum magnitude motion cannot be reduced below a certain value. Tolerances in the manufacture of the magnetic ring members and their incorporation into a static convergence structure will result in one ring member having slightly different properties than the other ring member, such as slightly different pole strengths and locations of the poles within the ring pair structure.
Another factor influencing the minimum magnitude of motion that a static convergence structure will produce is the location of the structure along the central Z-axis of the cathode ray tube. Because the velocity of the electron beams increases with increasing Z-axis distance from the electron guns, the magnetic fields of the static convergence device will produce different transverse magnetic forces depending where these fields are located along the Z-axis. Each ring member of a ring pair is located at a slightly different Z-axis location than is the other member. Thus, even if the ring members are rotated into a cancelling orientation, the magnetic field of one ring member, having a slightly different effect on the electron beam than the other, will not completely cancel the motion produced by the other ring member, this despite their similar structure. Static convergence structures located behind the G3 or G4 electrodes of the electron guns are therefore relatively more sensitive to the above-described velocity dependent effect than are static convergence devices further forward of the guns.
As thusly described for various reasons, regardless of the adjustments made to the static convergence device, a minimum magnitude motion of one electron beam relative to another will result. Since the electron beams will be subject to various conditions of misconvergence due to cathode ray tube manufacturing tolerances, converging an electron beam to within an acceptable distance of another will be possible for only selected conditions of misconvergence but not for all of the various conditions of misconvergence that may be encountered.
For example, a typical desirable capability of a static convergence device is to converge an outer beam onto the central beam to within 0.127mm distance. Consider then, illustratively, the appropriate ring pair of a static convergence device as described in the aforementioned Smith application. Because of the particular tolerances in device manufacture and Z-axis device location, the ring pair may produce a minimum magnitude motion of the outer beam of 0.762mm regardless of the rotational orientation of the ring members. Then, for those misconverged cathode ray tubes where the outer beam relative to the center beam is misconverged by 0.635mm or less, the static convergence device is unable, without additional structure, to converge the beams to within the required 0.127mm distance.
Additional structure associated with a static convergence device is described in U.S. patent application Ser. No. 667,834, filed Mar. 17, 1976, for R. L. Barbin. The purpose of this additional structure, illustratively used in conjunction with the static convergence device of the aforementioned U.S. patent of Barbin, is to provide for shortened operator setup time. The additional structure comprises a nonadjustable four-pole and a nonadjustable six-pole ring member fixedly located about a portion of the kinescope envelope. The fields produced by this arrangement move the outer beams in a predetermined direction relative to the center beam and place the outer beams in predetermined quadrants, regardless of the original misconverged locations of the beams. The operator will always know the manner in which to rotate the ring pairs of the remaining device structure, thereby shortening operator setup time. Such additional structure, however, is not suitable for enabling the static convergence device structure to converge electron beams to within an acceptable distance of each other for all of the various conditions of misconvergence that are encountered in cathode ray tube manufacture.